Conventionally, an information retrieval apparatus has been proposed as an apparatus that can present to a user a related word relevant to a search word entered by the user (for example, see Patent Reference 1). When the user enters the keyword, the information retrieval apparatus searches the number of cases in which a search formula specified by the entered keyword is established. Along with this, the information retrieval apparatus generates a related word for the search formula and obtains the number of cases in which a new search formula including the search formula and the generated related word is established, so as to present the number to the user.
In addition, a text classification apparatus has been suggested which allows automatic setting of a keyword for a second search only by specifying the keyword for a first search, which is entered by the user (for example, see Patent Reference 2). The sentence classification apparatus presents to the user, as a keyword for the second search, a word which is present in the same text as the keyword for the first search entered by the user and is different from the keyword for the first search present in other text, and which has a high appearance frequency.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 5-314182    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 10-162012